A camshaft is a curved mechanical part that functions to transmit energy from one element to another. The element that receives the energy is known as the follower and is present in close contact with the parent element. It is mainly used to open and close the valves in a piston engine. A camshaft is generally operated with the help of a crankshaft. The speed of a camshaft is approximately half the speed of a crankshaft. It can either exhibit rotary, oscillatory, or reciprocating motion. A camshaft mechanism comprises of three parts, i.e., a driver member known as cam, a driven member called the follower, and a supporting member, which is basically a frame or a structure built to support the cam and the follower. Camshafts are advantageous because they are capable of building both low-end and high-end torque as per the configuration. Also, they are used to improve the dynamic characteristics of the valve-actuating mechanism.
Construction of a Camshaft
A camshaft is usually curved in shape. The basic structure of the camshaft is generally constructed with the help of cast iron; however, there are other alternative elements used for its construction such as steel, stainless steel, brass, bronze, etc. Further, the induction hardening process is used to add strength to the structure of a camshaft and to protect it from wear and tear. For the purpose of induction hardening, the part to be heat traced is placed inside a copper coil. An alternating current is provided across the coil and the element is heated at a temperature above its transformation temperature.
Parts of a Camshaft
A camshaft assembly typically comprises of six main parts as given below:
1. Cam Bearing Journals
Cam bearing journals are the rotating parts of the camshaft that are present in close contact with the follower. It tends to transform the rotatory motion into linear motion.
2. Bearing Shells
A bearing shell is generally mounted on the top of the cam bearing journals. The purpose of bearing shells is to smoothen the rotation of a camshaft and to protect the engine from any sort of wear and tear in case of engine failure.
The main purpose of the lobes of a camshaft is to open and close the valves to allow proper fuel intake and waste discharge. The lobes work at a speed proportionate to the speed of the engine and tend to move in synchronisation with the motion of the piston.
4. Thrust Plate
A thrust plate is used to provide support to the cam at the rear of the engine. It is mounted upon the front cover and is attached between the cam and the rear gear.
5. Chain Sprocket
The chain sprocket is attached to the end of the engine where the camshaft is fixed. The purpose of a chain sprocket is to retain the timing belt during the working of the engine. The sprockets are further attached to the chain. The sprockets tend to move at a uniform speed even if they are not in contact with each other.
6. Woodruff Key
The woodruff key is one of the major components of a camshaft. The main objective of a woodruff key is to retain the movement of the cams at the perfect time.
Driving Mechanism of a Camshaft
A camshaft receives the driving power from a crankshaft. When a crankshaft rotates twice, the camshaft tends to undergo one rotation. The driving force is given to a camshaft via three means as given below:
1. Gear Drive
If there exists a minor distance between the camshaft and the crankshaft, a gear drive is used to power the camshaft. In such a case, the camshaft tends to rotate in the direction opposite to that of the direction of rotation of a crankshaft.
2. Chain Drive
A chain drive mechanism is used when there exists a significant distance between the camshaft and the crankshaft. A chain sprocket is used to transfer the energy from the crankshaft to the camshaft. Both the camshafts and crankshafts tend to rotate in the same direction. A chain tensioner is required to avoid the loosening of the chain system because a loose chain would not be able to transfer the appropriate amount of energy from a crankshaft to a camshaft.
3. Belt Drive
The belt drive mechanism is quite similar to that of a chain drive mechanism of powering a camshaft. The only difference is that here, a belt is used in place of a chain. A belt drive mechanism is generally preferred in the case of an overhead camshaft. To sustain the tension force in the belt, a belt tensioner is used.
Working of a Camshaft
The camshaft is operated with the help of a crankshaft, i.e., either by a pair of timing gears or a pair of timing sprockets connected to each other with the help of a chain mechanism. The pair of timing gears used to operate a camshaft is also known as a pair of meshing gears. The number of teeth present on the gear or sprocket of a camshaft is exactly equal to the number of teeth of the crankshaft gear or sprocket. This results in a 1:2 gear ratio therefore, the rotation speed of a camshaft is half that of the speed with which a crankshaft rotates. This means that when a crankshaft completes two rotations, the camshaft rotates once. A definite time relationship is maintained between the camshaft and the crankshaft with respect to the piston position with the help of gears and sprockets to ensure the opening and closing of valves at an accurate time.
Types of Camshafts
On the basis of lifters used by the camshaft
1. Flat Tappet Cam
Flat tappet cams consist of lifters that are flat shaped and have a slight crown on the face. The lobe of flat tappet cams is constructed in a slightly tapered shape. The tappet rides against the lobes of the camshaft. The tapered shape of the lobes helps the follower or the lifter to spin as it rides on the lobe. The flat tappet can be either a solid flat tappet or a hydraulic flat tappet. Flat tappet cams are generally made up of tool steel, hence they are light in weight and more durable than other types of camshafts.
2. Roller Cam
The roller cams comprise a wheel and axle arrangement. The wheel is attached to the base of the lifter body and is supported by needle bearings. The lobes of the roller camshafts are flat in shape. The roller camshafts are linked to each other so as to prevent them from rotating or spinning. Roller cams are advantageous because they reduce friction, thereby causing a significant increase in power. Also, the wear and tear rate of such cams are quite low. The engine efficiency can also be increased with the help of roller camshafts because it causes the valves to open at a faster rate and stay open for a longer duration, thereby allowing a proper intake and exhaust of fuel and air.
On the basis of the shape of the driver member of the camshaft
1. Disk or Plate Cam
A disk or a plate cam is also known as a radial cam. It consists of an irregular contour plate that is used to transfer motion to the driven member or the follower. The axis of the disk cam is perpendicular to the direction of motion of the follower.
2. Cylindrical Cam
A cylindrical cam is also known as a barrel of drum cam. As the name itself suggests, this type of cam is shaped like a cylinder. There exists a groove cut on the cylindrical cam surface. The follower tends to follow the groove path for movement. The axis of a cylindrical cam is parallel to the direction of motion of the driven member.
3. Translating Cam
The translating cam has a grooved contour plate. The follower or the driven member oscillates along the face of the cam grooving.
4. Wedge Cam
The structure of a wedge cam is wide at the base and has a pointed end. The follower tends to exhibit a sliding motion against the cam.
5. Spiral Cam
A spiral cam is a spiral semi-circular contour that has to groove on its face. The follower exhibits a vertical motion with respect to the reciprocating motion of the driver member of the camshaft or the cam.
6. Heart-shaped Cam
The structure of a heart-shaped cam is an asymmetrical heart. The follower moves along the edge of the cam. The motion exhibited by a heart-shaped cam is uniform and continuous. It is mainly used to evenly wound the wire on the former of a solenoid.
7. Spherical Cam
The spherical cam is shaped like a sphere that consists of a groove on its curved surface. The driven mechanism or the follower moves on the grooving. The movement of the follower is perpendicular to the axis of rotation of the spherical cam.
8. Conjugate Cam
A conjugate cam is provided with two rollers. Both the rollers are connected to the follower. The rollers tend to constrain the motion of each other, thereby eliminating noise. Such types of cams are used in applications where little or no noise is tolerable.
9. Globoid Cam
The structure of a globoid cam is similar to that of a cylindrical cam. The only difference is that the shape of the cylindrical shape here gets replaced by a convex or concave structure. The follower tends to move along the fulcrum.
There are a number of arrangements of camshafts on engines, out of which the following three configurations are most commonly used:
1. Single Overhead Cam
A single overhead cam represents the configuration in which the engine contains one cam per head. This means that an inline 4 cylinder or inline 6 cylinder engine consists of a single cam, while the V-6 and V-8 engine has two cams, one for each head. The cam tends to transfer motion to the rocker arms, which in turn presses down on the valves causing them to open. The closed position of the valve is achieved with the help of a spring that is attached to the rocker arms on one end and to the valve at the other end.
2. Double Overhead Cam
A double overhead configuration of cams represents an engine that has two cams per head. This means that the inline engines have two cams, while V engines have four. The engines that contain four or more valves per cylinder prefer to use a double overhead cam configuration. A double overhead cam increases the intake and exhaust capacity of the valves by expanding space for the gases and fuel to flow. This in turn increases the engine power. A single overhead cam used for an engine that contains two or more valves is unable to transfer motion to all the valves properly, hence is not usually preferred.
The camshaft of a pushrod engine is not present on the head, but instead, it is present inside the engine block. Such an engine makes use of long rods that go through the block into the head. The camshaft is driven with the help of gears or short chains. The function of the camshaft here is to transfer mechanical motion into the rods, which in turn move the rockers. The rockers open and close the valve accordingly. The long rods tend to add mass to the system, thereby increasing the load on the valves and decreasing the speed of pushrod engines. Hence, the pushrod configuration of a camshaft is used in applications where a limited speed of the engine is desired.
Examples of Camshafts
Camshafts are used in a number of applications. Some of them are listed below:
1. Operating the Valves of a System
The camshafts find their prime application in operating valves. The opening and closing of a valve highly depend on the movement of camshafts. The camshaft tends to move the rocker arms of the machine in an upward direction causing the valve to open. A spring attached to the valve then helps it to return back to the original position, thereby closing the valve.
2. Internal Combustion Engines
A camshaft is one of the most important parts of an internal combustion engine. They are used to open or close the inlet valve of the engine to control the amount of air and fuel entering the engine. They also allow the exhaust to properly get released from the combustion space. For each valve present in an engine, there exists a camshaft aligned at a particular angle. Generally, engines consist of a single camshaft; however, there exist some engines such as T head and V-8 engines that require two camshafts for their operation.
3. Operate various parts of an engine
A camshaft plays an important role to drive and power various parts of an engine such as an oil pump, fuel pump, and ignition distributor.